The present invention is related to high accuracy rotation sensing apparatus.
Magnetoresistive sensing apparatus are widely employed to discern rotational information for use in automotive powertrain control applications. Information such as cam shaft speed and absolute angular position are derived from voltage signals generated in response to resistive changes in one or more magnetoresistive (MR) elements placed in proximity to a permanent magnet and to a rotating target wheel or reluctor having predefined patterns of alternating high and low permeability regions.
In the simplest case, a single MR element may be used in generating a voltage signal which is compared to a reference voltage threshold which when crossed triggers output states indicating the proximity of high or low permeability regions of the reluctor. The transition edges of the output states correspond substantially to the end of one and beginning of another permeability region passing beneath the MR element. Such single element configurations generally do not provide the angular accuracy required in modern engine controls and suffer from well known temperature sensitivity shortfalls.
It is, however, common practice to employ a pair of matched MR elements in a differential signal processing mode to minimize or counteract the inherent temperature sensitivity effects of MR elements in general and particularly in the dynamic range of temperature experienced in internal combustion engine applications. Other benefits aside from temperature insensitivity are provided by such dual MR element configurations including improved angular accuracy. Furthermore, it is common practice to provide a buffer stage in front of a comparator stage having a set amount of hysteresis to provide robust noise immunity and rapid analog to digital signal conversion.
Two general differential signal processing configurations for a pair of MR elements include single and dual input amplification. In single input amplification, the input signal corresponds to the common node voltage between a pair of MR elements coupled in series between a voltage source. The input signal in this type of configuration directly corresponds to the resistance differential of the pair of MR elements and the amplification provides appropriate scaling and buffering of the signal. In dual input amplification, a two input differential amplifier receives an input signal at each input corresponding to voltage across a respective one of a pair of MR elements. Each MR element in this type of configuration is individually coupled in series with a matched current source. The input signals in this type of configuration correspond to the individual resistances of the pair of MR elements, and the amplification provides an appropriately scaled and buffered signal corresponding to the resistance differential of the pair of MR elements.
Subsequent to the single or dual input amplification, a comparator stage compares the scaled and buffered signal to a preset reference threshold. The reference threshold includes a conventional deadband or hysteresis band preset in accordance with anticipated noise amplitude or signal overshoot due to transition edge effect phenomenon.
Aside from the signal processing configurations described, two general configurations of MR element and target wheel interfacing are known. In a so called single track configuration, a pair of MR elements is disposed at the outer periphery of a rotating target wheel having a singular pattern of angularly alternating low and high permeability regions such that angular progression of the target wheel pattern is sensed first by one then by the other of the pair of MR elements. In this configuration, each MR element is angularly separated from the other with no axial offset along the rotational axis of the target wheel. In a so called dual track configuration, a pair of MR elements is disposed at the outer periphery of a rotating target wheel having a pair of axially separated complementary or mirror-image patterns of angularly alternating low and high permeability regions such that angular progression of the target wheel patterns are coincidentally sensed by the MR elements. In this configuration, each MR element is co-angular with respect to the axis and axial offset one from the other along the rotational axis of the target wheel. Such single and dual track configurations are disclosed for example in co-pending U.S. patent application Ser. No. 08/701,254 also assigned to the assignee of the present invention.
Generally, any combinationxe2x80x94including hybrids thereofxe2x80x94among the two general differential signal processing configurations and the two general configurations of MR element and target wheel interfacing described above may be employed. However, certain shortfalls are inherent in any combination of the known processing and hardware configurations. It is well known that the signal amplitude produced by MR based sensors varies significantly with the air gap between the MR elements and target wheel. Tight tolerancing of the air gap between the MR elements and the target wheel may be required to ensure that the signal amplitude will always be of appropriate magnitude for accurate and repeatable detection. In other words, the maximum air gap must be carefully controlled or the signal may not be detected to indicate rotational position of the target wheel. Similarly, the minimum air gap may be required to be carefully controlled or noise which scales proportionately with the signal amplitude may inappropriately indicate detection of a rotational position. For example, with respect to a single track configuration, signal overshoot due to transition edge effects being proportional to signal amplitude may cause erroneous rotational position indication if the air gap is too small. Additionally, the air gap tolerance band must be limited lest the slew rate differential between signals generated at opposite ends of the air gap tolerance band results in unacceptable angular inaccuracy of detected rotational position of the target wheel. In this regard, detection of small signals generated at large air gaps would lag detection of large signals generated at small air gaps and vice-versa.
In accordance with the present invention, a sensing apparatus detects transitions between relative states of a transducer. The transducer is characterized by a transducer output having an amplitude envelope which modulates with the operating region of the excitation stimulus. A comparator circuit has a pair of inputs into which a reference circuit provides a reference signal and a transducer circuit provides a transducer signal proportional to the transducer output. An hysteresis circuit adapts to the operating region of the transducer and provides an hysteretic deadband to the response characteristics of the comparator circuit proportional to the amplitude envelope.
The transducer may comprise one or more magnetoresistive elements responsive to the modulation of magnetic flux therethrough. Preferably, pairs of magnetoresistive elements are employed to take advantage of differential signal processing and attendant common mode cancellation effects.
In preferred implementations, magnetoresistive elements provide voltage signals to a voltage comparator. The voltage signal swings peak to peak in a characteristic fashion in response to the changes in flux density through the magnetoresistive elements. The peak to peak swings occur within an amplitude envelope which modulates in magnitude in accordance with the operating region of the flux density. A circuit, preferably including one of the magnetoresitive elements, provides an amount of hysteresis to the response characteristics of the comparator such that the hysteretic deadband is substantially proportional to the amplitude envelope of the voltage signal.